Yes, INERTIA is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, E 1, R 1, T 1, I 1, A 1.

Is it a valid word?

INERTIA is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a disposition to remain inactive or inert
(physics) the tendency of a body to maintain its state of rest or uniform motion unless acted upon by an external force

Not playable in Scrabble US: INACTIVENESS.